In modern, wide-field and/or laser-scanning microscopy with biological or other samples, it is essential that optical planes at different sample depths (z stacks) are quickly captured at a high numerical aperture (NA>0.8 with water immersion).
There are various different solutions for changing the focus in the known prior art. One of the solutions is the movement of the entire microscope objective. This method transmits vibrations to the sample if an immersion medium is used, such as water or oil, for example. In addition, the speed which can be achieved is limited by the high mass of the microscope objective.
The use of adaptive elements for changing the focus is also known. In this case, it is disadvantageous that it is necessary to insert an additional optical arrangement into the beam path to constitute the pupil. This is also very unwieldy.
Moreover, it is not possible to correct field aberrations in wide-field microscopy, such that the focal range of this solution is very restricted.
Another known solution is a movement of lenses in the tube optics used. Because of the low numerical aperture in the tube optics and the resulting low depth imaging factor, long movement paths are necessary in this configuration. In addition, aberrations arising on the lenses close to the object due to defocus are difficult or sometimes even impossible to correct at high numerical apertures.
In addition, a solution is known in which “Alvarez elements” are used for changing the focus. These are specially ground plates which generate a focal shift when moved toward each other. A dedicated access to the pupil is also necessary in this case. This positioning can be achieved in the objective due to the Alvarez plates functioning in the transmission path. However, this increases the technical complexity of the microscope objective significantly.
The typical approach for photographic objectives of moving lenses axially in the inside of the objective has led in the past to relatively little success. Due to the high numerical aperture, as well as the wide angle of incident light rays at the edge of the objective, strong aberrations are typically produced—in addition to a focal shift—when the lenses are moved. To correct these, a movement of additional lenses is necessary.
For this reason, in typical correction objectives with a long movement path, multiple lenses are typically moved. However, these coupled movements cannot be carried out very quickly.
By way of example, DE 102005034441A1 describes a microscope objective having an optical frontal element, multiple optical elements which are spaced apart from the frontal element and from each other, and an adjusting device, wherein at least one of the optical elements can be moved along the optical axis by means of the adjusting device in such a manner that the position of the focus of the objective is displaced relative to the frontal element in the direction of the optical axis and/or an aberration of the objective caused by temperature is compensated.
In this solution, the action of focusing at a high numerical aperture is carried out with movable elements. In this case, a diffractive element is moved. A substantial disadvantage in this case is that light is scattered by diffraction at other orders than the functional order.
Proceeding from the solutions according to the prior art, the problem addressed by the invention is that of further advancing an arrangement for focusing a microscope objective, in such a manner that it is possible to quickly change the focus, using simple means, over a relatively large focal range while avoiding vibrations and aberrations.